Fix cloning of installer-framework repo with custom branch

Cloning with --depth 0 and then explicitly switching branches won't work
for non-default branches. Instead directly check out the right branch.

Change-Id: I745d9d2224c69b3ba83b10f7f20129971f40f36d
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>